1. Field of the Invention
The present invention relates generally to image scanning apparatuses utilized for copying apparatuses or image readers and, more particularly, to an image scanning apparatus for scanning an original by forward movement of a scanning system reciprocated by a motor, thereby to expose an image of the original or read the image.
2. Description of the Background Art
Recently, more miniaturized and less expensive copying apparatuses and image readers are in demand as operation speed thereof is increasingly enhanced. Accordingly, in order to scan an image at a predetermined speed, it is desirable to reduce distance and elapsed time which are required for a scanning system to attain a predetermined scanning speed and to stop after terminating the scanning. It is also desirable to reduce a time period required for the scanning system to return to a home position after stopping the operation. Thus, a drive motor of the scanning system is required to transfer from an abrupt acceleration control to a constant speed control and from an abrupt deceleration control utilizing regenerative breaking applied by a counter-electromotive voltage and forcible breaking applied by a reverse drive through a stopped state to an abrupt reverse acceleration control.
FIG. 13 is a perspective view showing a detailed structure of an encoder attached to an axis of the motor for driving the scanning system. Referring to FIG. 13, a plurality of openings 74 are formed with a predetermined spacing in a circumferential direction on a disc-shaped encoder 33 fixed onto a rotary axis 30a. A light emitting element 70 and a light receiving element 72 are provided at a position corresponding to one of openings 74 with encoder 33 interposed therebetween. In this structure, one pulse is generated every time one of openings 74 passes in front of light emitting element 70 in accordance with the rotation of the motor.
The applicant of this application has already proposed an image scanning apparatus structured such that a predetermined rotation state of the motor is determined based on whether or not encoder pulses generated dependently on the rotation speed of the motor become a predetermined state, thereby to accomplish the above predetermined controls.
However, every time the motor stops, angular positions where the openings of the encoder stop shift little by little in practice. This sometimes results in such a case that one of the openings of the encoder stops overlapping a little with a detection position between light emitting element 70 and light receiving element 72 for detecting the shift in angular position. In this case, the width of an encoder pulse generated at an initial time of a stand-by state where the motor is driven again, that is, an initial time of the abrupt acceleration control becomes smaller corresponding to the overlapping of the opening of the encoder with the detection position (see l1 of FIG. 14). The conventional scanning apparatus determines that this pulse width results from high speed rotation of the motor. The pulse width determines whether or not the control is changed to the constant speed control. Therefore, the control may be changed to the constant speed control even though the motor does not yet attain a predetermined rotation speed.
Further, in order to change the abrupt deceleration control following the constant speed control to the abrupt reverse acceleration control through the stopped state, such a positive attempt is sometimes made to propel the abrupt deceleration control to the utmost possible limit to a time point when the control is changed to the abrupt reverse acceleration control. When the control is changed to the abrupt reverse acceleration control through the stopped state of the motor by detection of a pulse width corresponding to a speed at which the motor very nearly stops, the motor is sometimes reversely rotated as an expansion of forcible breaking action with the pulse width still undetected, in accordance with a deceleration method employing the regenerative breaking applied by a counter-electromotive voltage and forcible breaking applied by a reverse drive. In this case, if a portion of an opening of the encoder overlaps a little with the detection position between light emitting element 70 and light receiving element 72 and then the motor is immediately reversely rotated or accelerated after the reverse rotation, the width of a pulse to be detected becomes smaller, resulting in a determination that the motor is in a high speed operation state. Thus, a control for increasing the forcible breaking action is carried out to enhance the deceleration of the motor, resulting in reckless driving of the motor.